When you can't mix them. My wife and I are and oddity in this regard. I was a flaming lib when we met; she was laregly apolitical(perhaps non-practicing conservative would be another way to put it). She held conservative beliefs but didn't follow politics all that closely. We met as I was beginning my transition to conservatism. So I have a difficult time relating to this.
But I do know several couples right now who are divided ideologically, but happily married. For the life of me, I can't figure this out. The husand in one of these couples often jokes about making sure to vote and cancel out his wife's vote. If I didn't do what I do, I don't know that my wife and I would even talk politics as much as we do.
Obviously there is a lot more to a loving marriage than sharing an ideology. But I'm pretty sure that if I were in a place in my life where I was dating, that I too would probably seek out conservatives only.